1. Field of the Invention
This invention relates to a knee prosthesis for fitting to a patient as a replacement knee joint.
2. Related Art
Modern total knee replacement involves the resurfacing of the femoral condyles with a metallic component, roughly approximating to the shape of the anatomical femoral condyles, and resurfacing the tibial plateau with a polyethylene component having a metallic base plate.
One problem with such total knee replacement relates to the matching of the interior surface of the femoral component of the implant with the resected distal anterior surfaces of the femur. FIG. 1 shows a femoral bone, and from this it can be seen that the lateral femoral condyle (LFC) is longer than the medial femoral condyle (MFC) and that the condyles blend with the shaft of the femur in a different way on each side. A line connecting the points AM-AL makes an angle to the line connecting the points PM-PL. This presents the surgeon with a problem. Traditionally the distal end of the femur is cut with a cutting block and oscillating saw and generally five flat cuts are made. It used to be normal practice to insert the femoral component parallel to the line PM-PL and it can be seen that when a line parallel to this is made anteriorly, then AM and AL do not match the anterior cut. It has been discovered over the years that by externally rotating the cuts, and thus the femoral component, along the line EM-EL, the patellar track tends to be placed more in line with the normal patellar track P-P, and hence there is a feeling that better patellar tracking is achieved with an externally rotated femoral component. This provides a problem with the anterior cut because since the anterior cut in conventional Total Knee Replacements has to be parallel to the posterior cut EM-EL, then there is really no way of accurately matching up the bony landmarks AM-AL to the externally rotated anterior cut. The surgeon has two options. Firstly he can clear the bone at point AL and leave a gap medially between AM and the undersurface of the medial flare of the prosthesis. Alternatively, he can bring the anterior cut down to AM and make a notch into the lateral femur at point AL. This has the serious undesirable feature that digging a notch in the bone weakens the femur, causes a stress riser and, particularly in elderly ladies with brittle bones, risks a supra-condylar fracture of the femur.